The processes involved in papermaking have not changed appreciably in many years. In essence, the process of papermaking includes the steps of forming the paper fibers into a matted sheet and commencing to dewater the sheet, pressing the matted sheet through rollers to continue the dewatering process and to give the consolidated sheet its desired texture, and further drying the sheet as necessary to remove any remaining excess water from the sheet. Consequently, a papermaking machine generally includes three pertinent sections: the forming section, the press section and the dryer section.
The procedure of papermaking begins in the forming section with the preparation of a pulp slurry which is approximately 99 percent water and about 1 percent fiber. The pulp slurry is initially carried through the forming section of the papermaking machine on a forming fabric, not unlike a porous conveyor belt, where the pulp slurry is formed into a sheet. In the forming section, some water is removed, and the sheet is formed and transported to the press section of the papermaking machine where the process of removing the water from the sheet, begun in the forming section, is continued.
In the press section of a papermaking machine, the wet, matted sheet of paper fibers is transported on one or more press fabrics and is passed through at least one set of two rollers (and oftentimes a series of rollers) along with the press fabrics such that, in the press nip, at least some of the remaining water is squeezed out of the sheet and is absorbed through the permeable press fabric. As compression is increased between the rollers, water removal is likewise increased. The function of pressing also consolidates the sheet and provides texture to the surface of the sheet.
Although water content of the sheet is somewhat related to the type of papermaking machine employed and the sheet grade, typical consistency of the sheet of paper fibers as it enters the press section is about 20 percent fiber and about 80 percent water and at the end of the section is about 40 percent fiber and about 60 percent water. It will be appreciated, however, that the amount of water that the press fabric can absorb or carry away from the matted sheet is affected by the air and water permeability of the felt and the void volume of the felt, that being the volume that is not occupied by fibers or yarns. To that end, an important characteristic of press felts during operation is the ability of the felt to maintain void volume under load. Other significant press felt or fabric properties include resistance to abrasion, resistance to compaction, heat and chemical resistance as well as strength, permeability and caliper retention.
Within the press section, the sheet is supported and transported via one or more fabrics referred to as "press fabrics" or "press felts", which terms are used interchangeably in the industry. Therefore, unless otherwise specified, for purposes of this invention, the terms "press fabrics" or "press felts" as used herein shall refer to those fabrics used in the press section of a papermaking machine to support and transport the formed sheet of paper fibers to the dryer section of the machine where even more water may be removed.
Today, a press felt generally comprises a base fabric (e.g., a woven or non-woven cloth) having a staple fiber batt needle punched to it. In many press felts, multiple layers of batt fibers are needle punched to the paper side of the base cloth. In other embodiments, layers of batt fibers are needle punched onto each side of the base cloth. As yet another alternative, it is believed that some future commercial press felts may use no batt whatsoever, although most press felts can readily be distinguished from forming fabrics or dryer fabrics due to the presence of the layers of batt fibers. Likewise, it is possible that future press felts may contain no base fabric, these press felts simply comprising layers of batt fibers.
Currently, the base fabric of most press felts are made of 100 percent synthetics, primarily nylon polymers, although polyester and other materials have been used. It will be appreciated that the term "base fabric" refers to the underlying substrate of the press felt and includes scrim and composite structures as well as those woven and non-woven fabrics well known in the art as being suitable for use in press felts for papermaking machinery. Base fabrics are usually woven or otherwise constructed with cabled monofilaments, plied multifilaments, spun yarns or single monofilaments. They may be used in a single layer or multilayer mesh, and can be woven as endless belts or woven flat and joined with seams. The weave of the base fabric is often engineered to manipulate pressure uniformity, flow resistance, void volume and compression properties. These base fabrics may generally be classified as conventional (endless) designs, stratified (laminated) designs, and seam fabrics, and the monofilaments or fibers used therein are typically round in cross-section, although some patents have suggested using flat monofilaments, such as in Jackson U.S. Pat. No. 5,089,324, or oval monofilaments, such as in Marchand U.S. Pat. No. 5,651,394. Cunnane, III et al. U.S. Pat. No. 5,368,696 has also disclosed the use of hollow monofilaments as the fibers used in the base fabric. Alternatively, the base fabric may be a scrim, e.g., an extruded netting, or a composite structure, e.g., an extruded spun-bonded sheet, both of these types of substrates falling within the scope of the claimed invention.
The batt is also typically made from nylon fibers or other similar synthetic materials, which fibers are conventionally round in cross section. It will be appreciated that, for the purposes of the present invention, the term "batt" refers to essentially any kind of assembly or web of fibers other than the base fabric which is suitable for use in press felts, and is not necessarily limited to conventional batting. The fibers usually are carded into a uniform web to form the batt before being needle punched onto the base fabric, generally in a series of layers. Moreover, the batt fibers are often needle punched into the base fabric with the fibers oriented in the cross machine direction or in the machine direction, although alternative methods for needle punching now exist. The needling process can be engineered to affect the density, surface properties and permeability of the press fabric.
Upon review of the prior art with respect to press felts, it will be appreciated that the existing patent literature has generally focused on improving the base fabric of the press felts of papermaking machine in attempting to provide improved dewatering characteristics to the press felts. Very little, if any, literature has focused on or suggested improving the batt or the fibers thereof. In fact, most of the literature suggests doing away with the batt entirely, rather than seeking to improve its dewatering characteristics.
However, in the papermaking industry, improving the dewatering properties of the press felt is believed to be most desirable, regardless of which component of the press felt provides that improvement. Water removal costs far less in the press section than in the dryer section of a papermaking machine. Thus, the value of efficient press felt performance cannot be overemphasized. By improving the dewatering properties of a press felt only one percent, that is to say, for example, increasing the sheet dryness after pressing from 40 percent to 41 percent with a complementary decrease in the water content from 60 percent to 59 percent, a corresponding energy savings of about four percent is provided. Thus, less steam may, in turn, be used in the dryer section of the papermaking machine and/or the machine can run faster, if desired, resulting in increased production.
Using current technology, the round fibers used in the batt in current press felts permit some wicking from capillary action which is formed from the fiber to fiber voids. It will be appreciated that "wicking" is the ability, typically through capillary action, to carry or otherwise transport water or similar fluids from a previously saturated (i.e., wet) area of the press felt to a previously unsaturated (i.e., dry) area in order to provide the press felt with the ability to absorb or, more appropriately, to transport more water away from the sheet of paper fibers. Thus, by providing batt fibers having greater wicking action than the conventional fibers, it is believed that the press felts will have a greater ability to absorb water or transport water away from the sheet of paper fibers, resulting in increased dewatering performance. In turn, the more water that can be absorbed or taken away from the matted sheet of paper fibers, the greater the energy savings to the paper maker.
Thus, the need continues to exist for press felts having improved dewatering properties compared to conventional or currently employed press felts. Heretofore, the art with respect to press felts has not provided a way to improve these properties in the batt component of the press felts used in the press section of papermaking machines.